AI data center infrastructure stocks have shed a combined $4.2 trillion in market value since June, surrendering nearly half of the $9.4 trillion in gains accumulated over the prior 12 months, according to Financial Times analysis. The sell-off reflects growing uncertainty about the sustainability of the AI boom and concerns that data center expansion is slowing. Core suppliers have been hit hardest: HVAC installer Comfort Systems USA (FIX) and fiber-optic manufacturer Corning (GLW) have both halved from previous highs, while motion control maker Allient (ALNT) is down 35% from its October record. Cash flow pressure from massive AI capital expenditures by tech giants, expectations of local restrictions on new data center construction, and tighter interest rate policy are accelerating the valuation correction across the sector.